Relative roles of local and reflex components in cutaneous vasoconstriction during skin cooling in humans.

The reduction in skin blood flow (SkBF) with cold exposure is partly due to the reflex vasoconstrictor response from whole body cooling (WBC) and partly to the direct effects of local cooling (LC). Although these have been examined independently, little is known regarding their roles when acting together, as occurs in environmental cooling. We tested the hypothesis that the vasoconstrictor response to combined LC and WBC would be additive, i.e., would equal the sum of their independent effects. We further hypothesized that LC would attenuate the reflex vasoconstrictor response to WBC. We studied 16 (7 women, 9 men) young (30.5+/-2 yr) healthy volunteers. LC and WBC were accomplished with metal Peltier cooler-heater probe holders and water-perfused suits, respectively. Forearm SkBF was monitored by laser-Doppler flowmetry (LDF). Cutaneous vascular conductance (CVC) was calculated as LDF/blood pressure. Subjects underwent 15 min of LC alone or 15 min of WBC with and without simultaneous LC, either at equal levels (34-31 degrees C) or as equipotent stimuli (34-28 degrees C LC; 34-31 degrees C WBC). The fall in CVC with combined WBC and LC was greater (P<0.05) than for either alone (57.0+/-5% combined vs. 39.2+/-6% WBC; 34.4+/-4% LC) with equipotent cooling, but it was only significantly greater than for LC alone with equal levels of cooling (51.3+/-8% combined vs. 29.5+/-4% LC). The sum of the independent effects of WBC and LC was greater than their combined effects (74.9+/-4 vs. 51.3+/-8% equal and 73.6+/-7 vs. 57.0+/-5% equipotent; P<0.05). The fall in CVC with WBC at LC sites was reduced compared with control sites (17.6+/-2 vs. 42.4+/-8%; P<0.05). Hence, LC contributes importantly to the reduction in SkBF with body cooling, but also suppresses the reflex response, resulting in a nonadditive effect of these two components.     

Role of sensory nerves in the cutaneous vasoconstrictor response to local cooling in humans

Local cooling (LC) causes a cutaneous vasoconstriction (VC). In this study, we tested whether there is a mechanism that links LC to VC nerve function via sensory nerves. Six subjects participated. Local skin and body temperatures were controlled with Peltier probe holders and water-perfused suits, respectively. Skin blood flow at four forearm sites was monitored by laser-Doppler flowmetry with the following treatments: untreated control, pretreatment with local anesthesia (LA) blocking sensory nerve function, pretreatment with bretylium tosylate (BT) blocking VC nerve function, and pretreatment with both LA and BT. Local skin temperature was slowly reduced from 34 to 29 degrees C at all four sites. Both sites treated with LA produced an increase in cutaneous vascular conductance (CVC) early in the LC process (64 +/- 55%, LA only; 42 +/- 14% LA plus BT; P < 0.05), which was absent at the control and BT-only sites (5 +/- 8 and 6 +/- 8%, respectively; P > 0.05). As cooling continued, there were significant reductions in CVC at all sites (P < 0.05). At control and LA-only sites, CVC decreased by 39 +/- 4 and 46 +/- 8% of the original baseline values, which were significantly (P < 0.05) more than the reductions in CVC at the sites treated with BT and BT plus LA (-26 +/- 8 and -22 +/- 6%). Because LA affected only the short-term response to LC, either alone or in the presence of BT, we conclude that sensory nerves are involved early in the VC response to LC, but not for either adrenergic or nonadrenergic VC with longer term LC.     

Oral vitamin C enhances the adrenergic vasoconstrictor response to local cooling in human skin

Local administration of ascorbic acid (Asc) at a supraphysiological concentration inhibits the cutaneous vasoconstrictor response to local cooling (LC). However, whether orally ingesting Asc inhibits the LC-induced vasoconstrictor response remains unknown. The purpose of the present study was to examine the acute influence of oral Asc on the adrenergic vasoconstrictor response to LC in human skin. In experiment 1, skin blood flow (SkBF) was measured by laser-Doppler flowmetry at three sites (forearm, calf, palm). The three skin sites were locally cooled from 34 to 24°C at -1°C/min and maintained at 24°C for 20 min before (Pre) and 1.5 h after (Post) oral Asc (2-g single dose) or placebo supplementation. Cutaneous vascular conductance (CVC) was calculated as the ratio of SkBF to blood pressure and expressed relative to the baseline value before LC. Oral Asc enhanced (P < 0.05) the reductions in CVC in the forearm (Pre, -50.3 ± 3.3%; Post, -57.8 ± 2.2%), calf (Pre, -52.6 ± 3.7%; Post, -66.1 ± 4.3%), and palm (Pre, -46.2 ± 6.2%; Post, -60.4 ± 5.6%) during LC. The placebo did not change the responses at any site. In experiment 2, to examine whether the increased vasoconstrictor response caused by oral Asc is due to the adrenergic system, the release of neurotransmitters from adrenergic nerves in forearm skin was blocked locally by iontophoresis of bretylium tosylate (BT). Oral Asc enhanced (P < 0.05) the reductions in CVC at untreated control sites but did not change the responses at BT-treated sites during LC. In experiment 3, to further examine whether adrenergically mediated vasoconstriction is enhanced by oral Asc, 0.1 mM tyramine was administered using intradermal microdialysis in the forearm skin at 34°C in the Pre and Post periods. Oral Asc increased (P < 0.05) the tyramine-induced reduction in CVC. These findings suggest that oral Asc acutely enhances the cutaneous vasoconstrictor responses to LC through the modification of adrenergic sympathetic mechanisms.     

Cutaneous vasoconstrictor response to whole body skin cooling is altered by time of day.

To test for a diurnal difference in the vasoconstrictor control of the cutaneous circulation, we performed whole body skin cooling (water-perfused suits) at 0600 (AM) and 1600 (PM). After whole body skin temperature (T(sk)) was controlled at 35 degrees C for 10 min, it was progressively lowered to 32 degrees C over 18-20 min. Skin blood flow (SkBF) was monitored by laser-Doppler flowmetry at three control sites and at a site that had been pretreated with bretylium by iontophoresis to block noradrenergic vasoconstriction. After whole body skin cooling, maximal cutaneous vascular conductance (CVC) was measured by locally warming the sites of SkBF measurement to 42 degrees C for 30 min. Before whole body skin cooling, sublingual temperature (T(or)) in the PM was significantly higher than that in the AM (P < 0.05), but CVC, expressed as a percentage of maximal CVC (%CVC(max)), was not statistically different between AM and PM. During whole body skin cooling, %CVC(max) levels at bretylium-treated sites in AM or PM were not significantly reduced from baseline. In the PM, %CVC(max) at control sites fell significantly at T(sk) of 34.3 +/- 0.01 degrees C and lower (P < 0.05). In contrast, in the AM %CVC(max) at control sites was not significantly reduced from baseline until T(sk) reached 32.3 +/- 0.01 degrees C and lower (P < 0.05). Furthermore, the decrease in %CVC(max) in the PM was significantly greater than that in AM at T(sk) of 33.3 +/- 0.01 degrees C and lower (P < 0.05). Integrative analysis of the CVC response with respect to both T(or) and T(sk) showed that the cutaneous vasoconstrictor response was shifted to higher internal temperatures in the PM. These findings suggest that during whole body skin cooling the reflex control of the cutaneous vasoconstrictor system is shifted to a higher internal temperature in the PM. Furthermore, the slope of the relationship between CVC and T(sk) is steeper in the PM compared with that in the AM.     

Acetylcholine-induced vasodilation is mediated by nitric oxide and prostaglandins in human skin.

Acetylcholine (ACh) can effect vasodilation by several mechanisms, including activation of endothelial nitric oxide (NO) synthase and prostaglandin (PG) production. In human skin, exogenous ACh increases both skin blood flow (SkBF) and bioavailable NO levels, but the relative increase is much greater in SkBF than NO. This led us to speculate ACh may dilate cutaneous blood vessels through PGs, as well as NO. To test this hypothesis, we performed a study in 11 healthy people. We measured SkBF by laser-Doppler flowmetry (LDF) at four skin sites instrumented for intradermal microdialysis. One site was treated with ketorolac (Keto), a nonselective cyclooxygenase antagonist. A second site was treated with NG-nitro-L-arginine methyl ester (L-NAME) to inhibit NO synthase. A third site was treated with a combination of Keto and L-NAME. The fourth site was an untreated control site. After the three treated sites received the different inhibiting agents, ACh was administered to all four sites by intradermal microdialysis. Finally, sodium nitroprusside (SNP) was administered to all four sites. Mean arterial pressure (MAP) was monitored by Finapres, and cutaneous vascular conductance (CVC) was calculated (CVC = LDF/MAP). For data analysis, CVC values for each site were normalized to their respective maxima as effected by SNP. The results showed that both Keto and L-NAME each attenuated the vasodilation induced by exogenous ACh (ACh control = 79 +/- 4% maximal CVC, Keto = 55 +/- 7% maximal CVC, L-NAME = 46 +/- 6% maximal CVC; P < 0.05, ACh vs. Keto or L-NAME). The combination of the two agents produced an even greater attenuation of ACh-induced vasodilation (31 +/- 5% maximal CVC; P < 0.05 vs. all other sites). We conclude that a portion of the vasodilation effected by exogenous ACh in skin is due to NO; however, a significant portion is also mediated by PGs.     

Role of nitric oxide in the vascular effects of local warming of the skin in humans.

Local warming of skin induces vasodilation by unknown mechanisms. To test whether nitric oxide (NO) is involved, we examined effects of NO synthase (NOS) inhibition with NG-nitro-L-arginine methyl ester (L-NAME) on vasodilation induced by local warming of skin in six subjects. Two adjacent sites on the forearm were instrumented with intradermal microdialysis probes for delivery of L-NAME and sodium nitroprusside. Skin blood flow was monitored by laser-Doppler flowmetry (LDF) at microdialysis sites. Local temperature (Tloc) of the skin at both sites was controlled with special LDF probe holders. Mean arterial pressure (MAP; Finapres) was measured and cutaneous vascular conductance calculated (CVC = LDF/MAP = mV/mmHg). Data collection began with a control period (Tloc at both sites = 34 degrees C). One site was then warmed to 41 degrees C while the second was maintained at 34 degrees C. Local warming increased CVC from 1.44 +/- 0.41 to 4.28 +/- 0.60 mV/mmHg (P < 0.05). Subsequent L-NAME administration reduced CVC to 2.28 +/- 0.47 mV/mmHg (P < 0.05 vs. heating), despite the continued elevation of Tloc. At a Tloc of 34 degrees C, L-NAME reduced CVC from 1.17 +/- 0.23 to 0.75 +/- 0.11 mV/mmHg (P < 0.05). Administration of sodium nitroprusside increased CVC to levels no different from those induced by local warming. Thus NOS inhibition attenuated, and sodium nitroprusside restored, the cutaneous vasodilation induced by elevation of Tloc; therefore, the mechanism of cutaneous vasodilation by local warming requires NOS generation of NO.     

Neuropeptide Y antagonism reduces reflex cutaneous vasoconstriction in humans

Previous studies have provided evidence of a non-noradrenergic contributor to reflex cutaneous vasoconstriction in humans but did not identify the transmitter responsible. To test whether neuropeptide Y (NPY) has a role, in two series of experiments we slowly reduced whole body skin temperature (TSK) from 34.5 to 31.7 degrees C. In protocol 1, Ringer solution and the NPY receptor antagonist BIBP-3226 alone were delivered intradermally via microdialysis. In protocol 2, yohimbine plus propranolol (Yoh + Pro), Yoh + Pro in combination with BIBP-3226, and Ringer solution were delivered to antagonize locally the vasomotor effects of NPY and norepinephrine. Blood flow was measured by laser Doppler flowmetry (LDF). Mean arterial blood pressure (MAP) was monitored at the finger (Finapres). In protocol 1, cutaneous vascular conductance (CVC) fell by 45%, to 55.1 +/- 5.6% of baseline at control sites (P < 0.05). At BIBP-3226-treated sites, CVC fell by 34.1% to 65.9 +/- 5.0% (P < 0.05; P < 0.05 between sites). In protocol 2, during body cooling, CVC at control sites fell by 32.6%, to 67.4 +/- 4.3% of baseline; at sites treated with Yoh + Pro, CVC fell by 18.7%, to 81.3 +/- 4.4% of baseline (P < 0.05 vs. baseline; P < 0.05 vs. control) and did not fall significantly at sites treated with BIBP-3226 + Yoh + Pro (P > 0.05; P < 0.05 vs. other sites). After cooling, exogenous norepinephrine induced vasoconstriction at control sites (P < 0.05) but not at sites treated with Yoh + Pro + BIBP-3226 (P > 0.05). These results indicate that NPY participates in sympathetically mediated cutaneous vasoconstriction in humans during whole body cooling.     

Cutaneous vascular responses to isometric handgrip exercise

Cutaneous vascular responses to dynamic exercise have been well characterized, but it is not known whether that response pattern applies to isometric handgrip exercise. We examined cutaneous vascular responses to isometric handgrip and dynamic leg exercise in five supine men. Skin blood flow was measured by laser-Doppler velocimetry and expressed as laser-Doppler flow (LDF). Arterial blood pressure was measured noninvasively once each minute. Cutaneous vascular conductance (CVC) was calculated as LDF/mean arterial pressure. LDF and CVC responses were measured at the forearm and chest during two 3-min periods of isometric handgrip at 30% of maximum voluntary contraction and expressed as percent changes from the preexercise levels. The skin was normothermic (32 degrees C) for the first period of handgrip and was locally warmed to 39 degrees C for the second handgrip. Finally, responses were observed during 5 min of dynamic two-leg bicycle exercise (150-175 W) at a local skin temperature of 39 degrees C. Arm LDF increased 24.5 +/- 18.9% during isometric handgrip in normothermia and 64.8 +/- 14.1% during isometric handgrip at 39 degrees C (P less than 0.05). Arm CVC did not significantly change at 32 degrees C but significantly increased 18.1 +/- 6.5% during isometric handgrip at 39 degrees C (P less than 0.05). Arm LDF decreased 12.2 +/- 7.9% during dynamic exercise at 39 degrees C, whereas arm CVC fell by 35.3 +/- 4.6% (in each case P less than 0.05). Chest LDF and CVC showed similar responses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cold-induced cutaneous vasoconstriction is mediated by Rho kinase in vivo in human skin

Cutaneous vasoconstriction (VC) is the initial thermoregulatory response to cold exposure and can be elicited through either whole body or localized skin cooling. However, the mechanisms governing local cold-induced VC are not well understood. We tested the hypothesis that Rho kinase participates in local cold-induced cutaneous VC. In seven men and women (20-27 yr of age), up to four ventral forearm skin sites were instrumented with intradermal microdialysis fibers for localized drug delivery during cooling. Skin blood flow was monitored at each site with laser-Doppler flowmetry while local skin temperature was decreased and maintained at 24 degrees C for 40 min. Cutaneous vascular conductance (CVC; laser-Doppler flowmetry/mean arterial pressure) was expressed as percent change from 34 degrees C baseline. During the first 5 min of cooling, CVC decreased at control sites (lactated Ringer solution) to -45 +/- 6% (P < 0.001), increased at adrenoceptor-antagonized sites (yohimbine + propranolol) to 15 +/- 14% (P = 0.002), and remained unchanged at both Rho kinase-inhibited (fasudil) and adrenoceptor-antagonized + Rho kinase-inhibited sites (yohimbine + propranolol + fasudil) (-9 +/- 1%, P = 0.4 and -6 +/- 2%, P = 0.4, respectively). During the last 5 min of cooling, CVC further decreased at all sites when compared with baseline values (control, -77 +/- 4%, P < 0.001; adrenoceptor antagonized, -61 +/- 3%, P < 0.001; Rho kinase inhibited, -34 +/- 7%, P < 0.001; and adrenoceptor antagonized + Rho kinase inhibited sites, -35 +/- 3%, P < 0.001). Rho kinase-inhibited and combined treatment sites were significantly attenuated when compared with both adrenoceptor-antagonized (P < 0.01) and control sites (P < 0.0001). Rho kinase mediates both early- and late-phase cold-induced VC, supporting in vitro findings and providing a putative mechanism through which both adrenergic and nonadrenergic cold-induced VC occurs in an in vivo human thermoregulatory model.     

Bradykinin does not mediate cutaneous active vasodilation during heat stress in humans.

To test the hypothesis that bradykinin effects cutaneous active vasodilation during hyperthermia, we examined whether the increase in skin blood flow (SkBF) during heat stress was affected by blockade of bradykinin B(2) receptors with the receptor antagonist HOE-140. Two adjacent sites on the forearm were instrumented with intradermal microdialysis probes for local delivery of drugs in eight healthy subjects. HOE-140 was dissolved in Ringer solution (40 microM) and perfused at one site, whereas the second site was perfused with Ringer alone. SkBF was monitored by laser-Doppler flowmetry (LDF) at both sites. Mean arterial pressure (MAP) was monitored from a finger, and cutaneous vascular conductance (CVC) was calculated (CVC = LDF/MAP). Water-perfused suits were used to control body temperature and evoke hyperthermia. After hyperthermia, both microdialysis sites were perfused with 28 mM nitroprusside to effect maximal vasodilation. During hyperthermia, CVC increased at HOE-140 (69 +/- 2% maximal CVC, P < 0.01) and untreated sites (65 +/- 2% maximal CVC, P < 0.01). These responses did not differ between sites (P > 0.05). Because the bradykinin B(2)-receptor antagonist HOE-140 did not alter SkBF responses to heat stress, we conclude that bradykinin does not mediate cutaneous active vasodilation.     

Endothelial nitric oxide synthase control mechanisms in the cutaneous vasculature of humans in vivo

Nitric oxide (NO) participates in locally mediated vasodilation induced by increased local skin temperature (T(loc)) and in sympathetically mediated vasodilation during whole body heat stress. We hypothesized that endothelial NOS (eNOS) participates in the former, but not the latter, response. We tested this hypothesis by examining the effects of the eNOS antagonist N(G)-amino-l-arginine (l-NAA) on skin blood flow (SkBF) responses to increased T(loc) and whole body heat stress. Microdialysis probes were inserted into forearm skin for drug delivery. One microdialysis site was perfused with l-NAA in Ringer solution and a second site with Ringer solution alone. SkBF [laser-Doppler flowmetry (LDF)] and blood pressure [mean arterial pressure (MAP)] were monitored, and cutaneous vascular conductance (CVC) was calculated (CVC = LDF / MAP). In protocol 1, T(loc) was controlled with LDF/local heating units. T(loc) initially was held at 34 degrees C and then increased to 41.5 degrees C. In protocol 2, after a normothermic period, whole body heat stress was induced (water-perfused suits). At the end of both protocols, 58 mM sodium nitroprusside was perfused at both microdialysis sites to cause maximal vasodilation for data normalization. In protocol 1, CVC at 34 degrees C T(loc) did not differ between l-NAA-treated and untreated sites (P > 0.05). Local skin warming to 41.5 degrees C T(loc) increased CVC at both sites. This response was attenuated at l-NAA-treated sites (P < 0.05). In protocol 2, during normothermia, CVC did not differ between l-NAA-treated and untreated sites (P > 0.05). During heat stress, CVC rose to similar levels at l-NAA-treated and untreated sites (P > 0.05). We conclude that eNOS is predominantly responsible for NO generation in skin during responses to increased T(loc), but not during reflex responses to whole body heat stress.     

Reflex control of cutaneous vasoconstrictor system is reset by exogenous female reproductive hormones.

To determine whether cardiovascular influences of exogenous female steroid hormones include effects on reflex thermoregulatory control of the adrenergic cutaneous vasoconstrictor system, we conducted ramp decreases in skin temperature (T(sk)) in eight women in both high- and low (placebo)-progesterone/estrogen phases of oral contraceptive use. With the use of water-perfused suits, T(sk) was held at 36 degrees C for 10 min (to minimize initial vasoconstrictor activity) and was then decreased in a ramp, approximately 0.2 degrees C/min for 12-15 min. Subjects rested supine for 30-40 min before each experiment, and the protocol was terminated before the onset of shivering. Skin blood flow was monitored by laser-Doppler flowmetry and arterial pressure by finger photoplethysmography. In all experiments, cutaneous vasoconstriction began immediately with the onset of cooling, and cutaneous vascular conductance (CVC) decreased progressively with decreasing T(sk). Regression analysis of the relationship of CVC to T(sk) showed no difference in slope between phases (low-hormone phase: 17.67 +/- 5.57; high-hormone phase: 17.40 +/- 8.00 %baseline/ degrees C; P > 0.05). Additional studies involving local blockade confirmed this response as being solely due to the adrenergic vasoconstrictor system. Waking oral temperature (T(or)) was significantly higher on high-hormone vs. low-hormone days (36.60 +/- 0.11 vs. 36.37 +/- 0.09 degrees C, respectively; P < 0.02). Integrative analysis of CVC in terms of simultaneous values for T(sk) and T(or) showed that the cutaneous vasoconstrictor response was shifted in the high-hormone phase such that a higher T(or) was maintained throughout cooling (P < 0.05). Thus reflex thermoregulatory control of the cutaneous vasoconstrictor system is shifted to higher internal temperatures by exogenous female reproductive hormones.     

Selected contribution: Gender differences in the endothelin-B receptor contribution to basal cutaneous vascular tone in humans.

To test whether the contribution of endothelin-B (ET-B) receptors to resting vascular tone differs between genders, we administered the ET-B receptor antagonist BQ-788 into the forearm skin of 11 male and 11 female subjects by intradermal microdialysis. Skin blood flow was measured using laser-Doppler flowmetry at the microdialysis site. The probe was perfused with Ringer solution alone, followed by BQ-788 (150 nM) and finally sodium nitroprusside (28 mM) to effect maximal cutaneous vasodilation. Cutaneous vascular conductance (CVC) was calculated (laser-Doppler flowmetry/mean arterial pressure) and normalized to maximal levels (%max). In male subjects, baseline CVC was (mean +/- SE) 19 +/- 3%max and increased to 26 +/- 5%max with BQ-788 (P < 0.05 vs. baseline). In female subjects, baseline CVC was 13 +/- 1%max and decreased to 10 +/- 1%max in response to BQ-788. CVC responses to BQ-788 differed with gender (P < 0.05); thus the contribution of ET-B receptors to resting cutaneous vascular tone differs between men and women. In men, ET-B receptors mediate tonic vasoconstriction, whereas, in women, ET-B receptors mediate tonic vasodilation.     

Baroreceptor modulation of active cutaneous vasodilation during dynamic exercise in humans.

The hypothesis that baroreceptor unloading during dynamic limits cutaneous vasodilation by withdrawal of active vasodilator activity was tested in seven human subjects. Increases in forearm skin blood flow (laser-Doppler velocimetry) at skin sites with (control) and without alpha-adrenergic vasoconstrictor activity (vasodilator only) and in arterial blood pressure (noninvasive) were measured and used to calculate cutaneous vascular conductance (CVC). Subjects performed two similar dynamic exercise (119 +/- 8 W) protocols with and without baroreceptor unloading induced by application of -40 mmHg lower body negative pressure (LBNP). The LBNP condition was reversed (i.e., either removed or applied) after 15 min while exercise continued for an additional 15 min. During exercise without LBNP, the increase in body core temperature (esophageal temperature) required to elicit active cutaneous vasodilation averaged 0.25 +/- 0.08 and 0.31 +/- 0.10 degrees C (SE) at control and vasodilator-only skin sites, respectively, and increased to 0.44 +/- 0.10 and 0.50 +/- 0.10 degrees C (P < 0.05 compared with without LBNP) during exercise with LBNP. During exercise baroreceptor unloading delayed the onset of cutaneous vasodilation and limited peak CVC at vasodilator-only skin sites. These data support the hypothesis that during exercise baroreceptor unloading modulates active cutaneous vasodilation.     

Systemic hypoxia causes cutaneous vasodilation in healthy humans.

Hypoxia and hypercapnia represent special challenges to homeostasis because of their effects on sympathetic outflow and vascular smooth muscle. In the cutaneous vasculature, even small changes in perfusion can shift considerable blood volume to the periphery and thereby impact both blood pressure regulation and thermoregulation. However, little is known about the influence of hypoxia and hypercapnia on this circulation. In the present study, 35 healthy subjects were instrumented with two microdialysis fibers in the ventral forearm. Each site was continuously perfused with saline (control) or bretylium tosylate (10 mM) to prevent sympathetically mediated vasoconstriction. Skin blood flow was assessed at each site (laser-Doppler flowmetry), and cutaneous vascular conductance (CVC) was calculated as red blood cell flux/mean arterial pressure and normalized to baseline. In 13 subjects, isocapnic hypoxia (85 and 80% O(2) saturation) increased CVC to 120 +/- 10 and 126 +/- 7% baseline in the control site (both P < 0.05) and 113 +/- 3 (P = 0.087) and 121 +/- 4% baseline (P < 0.05) in the bretylium site. Adrenergic blockade did not affect the magnitude of this response (P > 0.05). In nine subjects, hyperpnea (matching hypoxic increases in tidal volume) caused no change in CVC in either site (both P > 0.05). In 13 subjects, hypercapnia (+5 and +9 Torr) increased CVC to 111 +/- 4 and 111 +/- 4% baseline, respectively, in the control site (both P < 0.05), whereas the bretylium site remained unchanged (both P > 0.05). Thus both hypoxia and hypercapnia cause modest vasodilation in nonacral skin. Adrenergic vasoconstriction of neural origin does not restrain hypoxic vasodilation, but may be important in hypercapnic vasodilation.     

Control of internal temperature threshold for active cutaneous vasodilation by dynamic exercise.

Exercise induces shifts in the internal temperature threshold at which cutaneous vasodilation begins. To find whether this shift is accomplished through the vasoconstrictor system or the cutaneous active vasodilator system, two forearm sites (0.64 cm2) in each of 11 subjects were iontophoretically treated with bretylium tosylate to locally block adrenergic vasoconstrictor control. Skin blood flow was monitored by laser-Doppler flowmetry (LDF) at those sites and at two adjacent untreated sites. Mean arterial pressure (MAP) was measured noninvasively. Cutaneous vascular conductance was calculated as LDF/MAP. Forearm sweat rate was also measured in seven of the subjects by dew point hygrometry. Whole body skin temperature was raised to 38 degrees C, and supine bicycle ergometer exercise was then performed for 7-10 min. The internal temperature at which cutaneous vasodilation began was recorded for all sites, as was the temperature at which sweating began. The same subjects also participated in studies of heat stress without exercise to obtain vasodilator and sudomotor thresholds from rest. The internal temperature thresholds for cutaneous vasodilation were higher during exercise at both bretylium-treated (36.95 +/- 0.07 degrees C rest, 37.20 +/- 0.04 degrees C exercise, P less than 0.05) and untreated sites (36.95 +/- 0.06 degrees C rest, 37.23 +/- 0.05 degrees C exercise, P less than 0.05). The thresholds for cutaneous vasodilation during rest or during exercise were not statistically different between untreated and bretylium-treated sites (P greater than 0.05). The threshold for the onset of sweating was not affected by exercise (P greater than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Nonnoradrenergic mechanism of reflex cutaneous vasoconstriction in men

We tested for a nonnoradrenergic mechanism of reflex cutaneous vasoconstriction with whole body progressive cooling in seven men. Forearm sites (<1 cm(2)) were pretreated with: 1) yohimbine (Yoh; 5 mM id) to antagonize alpha-adrenergic receptors, 2) Yoh plus propranolol (5 mM Yoh-1 mM PR id) to block alpha- and beta-adrenergic receptors, 3) iontophoretic application of bretylium tosylate (BT) to block all sympathetic vasoconstrictor nerve effects, or 4) intradermal saline. Skin blood flow was measured by laser Doppler flowmetry and arterial pressure by finger photoplethysmography; cutaneous vascular conductance (CVC) was indexed as the ratio of the two. Whole body skin temperature (T(SK)) was controlled at 34 degrees C (water-perfused suit) for 10 min and then lowered to 31 degrees C over 15 min. During cooling, vasoconstriction was blocked at BT sites (P > 0.05). CVC at saline sites fell significantly beginning at T(SK) of 33.4 +/- 0.01 degrees C (P <0.05). CVC at Yoh-PR sites was significantly reduced beginning at TSK of 33.0 +/- 0.01 degrees C (P < 0.05). After cooling, iontophoretic application of norepinephrine (NE) confirmed blockade of adrenergic receptors by Yoh-PR. Because the effects of NE were blocked at sites showing significant reflex vasoconstriction, a nonnoradrenergic mechanism in human skin is indicated, probably via a sympathetic cotransmitter.     

Nitric oxide concentration increases in the cutaneous interstitial space during heat stress in humans.

To examine the role of nitric oxide (NO) in cutaneous active vasodilation, we measured the NO concentration from skin before and during whole body heat stress in nine healthy subjects. A forearm site was instrumented with a NO-selective, amperometric electrode and an adjacent intradermal microdialysis probe. Skin blood flow (SkBF) was monitored by laser-Doppler flowmetry (LDF). NO concentrations and LDF were measured in normothermia and heat stress. After heat stress, a solution of ACh was perfused through the microdialysis probe to pharmacologically generate NO and verify the electrode's function. During whole body warming, both SkBF and NO concentrations began to increase at the same internal temperature. Both SkBF and NO concentrations increased during heat stress (402 +/- 76% change from LDF baseline, P < 0.05; 22 +/- 5% change from NO baseline, P < 0.05). During a second baseline condition after heat stress, ACh perfusion led to increases in both SkBF and NO concentrations (496 +/- 119% change from LDF baseline, P < 0.05; 16 +/- 10% change from NO baseline, P < 0.05). We conclude that NO does increase in skin during heat stress in humans, attendant to active vasodilation. This result suggests that NO has a role beyond that of a permissive factor in the process; rather, NO may well be an effector of cutaneous vasodilation during heat stress.     

Cyclooxygenase and nitric oxide synthase dependence of cutaneous reactive hyperemia in humans

We tested the hypothesis that cyclooxygenases (COXs) or COX products inhibit nitric oxide (NO) synthesis and thereby mask potential effects of NO on reactive hyperemia in the cutaneous circulation. We performed laser-Doppler flowmetry (LDF) with intradermal microdialysis in 12 healthy volunteers aged 19-25 yr. LDF was expressed as the percent cutaneous vascular conduction (%CVC) or as the maximum %CVC (%CVC(max)) where CVC is LDF/mean arterial pressure. We tested the effects of the nonisoform-specific NO synthase inhibitor nitro-L-arginine (NLA, 10 mM), the nonspecific COX inhibitor ketorolac (Keto, 10 mM), combined NLA + Keto, and NLA + sodium nitroprusside (SNP, 28 mM) on baseline and reactive hyperemia flow parameters. We also examined the effects of isoproterenol, a beta-adrenergic agonist that causes prostaglandin-independent vasodilation to correct for the increase in baseline flow caused by Keto. When delivered directly into the intradermal space, Keto greatly augments all aspects of the laser-Doppler flow response to reactive hyperemia: peak reactive hyperemic flow increased from 41 +/- 5 to 77 +/- 7%CVC(max), time to peak flow increased from 17 +/- 3 to 56 +/- 24 s, the area under the reactive hyperemic curve increased from 1,417 +/- 326 to 3,376 +/- 876%CVC(max).s, and the time constant for the decay of peak flow increased from 100 +/- 23 to 821 +/- 311 s. NLA greatly attenuates the Keto response despite exerting no effects on baseline LDF or on reactive hyperemia when given alone. Low-dose NLA + SNP duplicates the Keto response. Isoproterenol increased baseline and peak reactive flow. These results suggest that COX inhibition unmasks NO dependence of reactive hyperemia in human cutaneous circulation.     

Mechanisms of vasoactive intestinal peptide-mediated vasodilation in human skin.

Vasoactive intestinal peptide (VIP) is known to induce histamine release in human skin and to include a nitric oxide (NO)-dependent dilation in several other vascular beds. However, the relative contribution of histamine and NO to VIP-mediated vasodilation in human skin is unknown. Forty-three subjects volunteered to participate in two studies designed to examine the mechanism of VIP-mediated vasodilation in human skin. Study 1 examined the contribution of NO in the skin blood flow response to eight doses of VIP ranging from 25 to 800 pmol. In addition, study 1 examined a specific role for NO in VIP-mediated dilation. Study 2 examined the relative contribution of NO and histamine to VIP-mediated dilation via H1 and H2 histamine receptors. Infusions were administered to skin sites via intradermal microdialysis. Red blood cell flux was measured by using laser-Doppler flowmetry (LDF), and cutaneous vascular conductance (CVC; LDF/mean arterial pressure) was calculated and normalized to maximal vasodilation. VIP-mediated vasodilation includes a NO-dependent component at doses above 100 pmol, where NO synthase inhibition significantly attenuates CVC (P < 0.05). Inhibition of H1 receptors attenuates the rise in CVC to exogenous VIP (P < 0.05); however, combined H1-receptor inhibition and NO synthase inhibition further reduced VIP-mediated vasodilation compared with either H1 inhibition or NO synthase inhibition alone (P < 0.05). In contrast to H1-receptor inhibition, H2-receptor inhibition did not affect vasodilation to exogenous VIP. Thus, in human skin, VIP-mediated vasodilation includes a NO-dependent component that could not be explained by H1- and H2-receptor activation.